Coating precursor and method for coating a substrate with a refractory layer

ABSTRACT

The purpose of the invention is a coating precursor comprising a silicone resin and a mineral filler capable of chemically reacting so as to produce a solid layer on a substrate and a cohesive refractory layer after a calcination operation. The precursor may also include an additive capable of reducing its viscosity. Another purpose of the invention is a method for coating a given surface of a substrate with at least one refractory layer containing silicon in which the substrate is coated with a coating precursor according to the invention so as to form a green layer and a heat treatment is carried out to calcine the said green layer and to form a cohesive refractory layer. The invention is a means of obtaining a protective coating capable of resisting oxidising environments, liquid metal and a solid or molten salt.

FIELD OF THE INVENTION

[0001] This invention relates to the protection of objects and materialsfor use in the metallurgical industry, and particularly the aluminiumindustry. In particular, it relates to protective coatings for the saidobjects and materials.

STATE OF THE ART

[0002] The objects and materials used in the aluminium industry arefrequently exposed to corrosive environments and subjected to hightemperatures and severe thermal constraints. Containers (such as ladlesand furnaces), ducts (such as troughs, injectors and casting nozzles)and tools and devices designed to handle and treat liquid aluminium(such as filters and rotors) must have good mechanical and chemicalstrength. In particular, the surfaces of these objects that are exposedto liquid aluminium must not dissolve in liquid aluminium norcontaminate it.

[0003] Although the strength of materials typically used in thealuminium industry is generally sufficient, there are some applicationsor conditions for which an even higher strength is required. This is thecase particularly when it is required to reduce the number of inclusionsper tonne of cast aluminium to almost zero.

[0004] Therefore, the applicant looked for means of handling, producing,treating and casting aluminium and liquid aluminium alloyssatisfactorily under the most demanding conditions and applications.

DESCRIPTION IF THE INVENTION

[0005] An object of the invention is a coating precursor to be used forthe formation of a protective layer on a substrate. The said precursorcomprises a silicone resin (or organosiloxane) and a mineral fillercapable of reacting chemically with the said resin so as to produce acohesive refractory layer after a calcination operation of the layer.

[0006] The said precursor, which is typically in the form of a powder,is preferably homogenous.

[0007] The silicone resin is a polysiloxane preferably comprising aproportion of OH groups, such as a polymethylsiloxane, apolydimethylsiloxane, a polymethylsilsesquioxane, or a mixture thereof,comprising a proportion of OH groups substituted for methyl groups. Theapplicant has noted that the proportion of OH groups is preferablybetween about 0.5% and about 2%. If the proportion of OH groups is toolow, there will not be sufficient propension to form a solid layer withgood cohesiveness after calcination. A very high proportion of OH groupsmay make the polysiloxane difficult to produce at an acceptable cost.The silanol (Si—OH) groups are preferably stable so that the resin canbe stored. These OH groups may be grafted to a polysiloxane byhydrolysis. The siloxanic patterns of the polysiloxane according to theinvention are advantageously wholly or partly trifunctional orquadrifunctional.

[0008] The mineral filler is typically chosen from among metal borides,carbides, nitrides and oxides, or from among non-metal borides, carbidesand nitrides (such as boron nitrides) or a combination or a mixturethereof. The said mineral filler is advantageously chosen from amongmetal compounds such as metal oxides, metal carbides, metal borides andmetal nitrides, or a combination or a mixture thereof. The mineralfiller is preferably capable of chemically reacting with the siliconeresin so as to produce a refractory layer with good cohesiveness aftercalcination of the said green layer. The mineral filler may be chosen asa function of the physicochemical characteristics expected from thecoating (such as wettability or non-wettability by a liquid metal).

[0009] The metal compound is advantageously alumina, ZrO₂, ZrB₂, TiB₂ orTiO₂ or a combination or a mixture thereof. The alumina is preferably areactive calcined alpha alumina called a technical alumina, with a verylow hydration ratio (typically less than 1%, or even less than 0.5%).

[0010] The mineral filler is preferably in the form of a powder. Thesize grading of the mineral filler powder is typically such that thesize of the grains is between 1.5 μm and 100 μm.

[0011] The physical properties of the coating, such as its mechanicalproperties (including resistance to thermal shock), can sometimes beadapted by adjusting the proportion of the mineral filler and/or itssize grading.

[0012] The proportion of silicone resin in the precursor is typicallybetween 10 and 20% by weight, to enable satisfactory ceramisation of thecoating during calcination.

[0013] The proportion of the mineral filler in the precursor istypically between 80% and 90% by weight.

[0014] According to an advantageous variant of this invention, theprecursor also includes an additive capable of reducing the viscosity ofthe precursor. The said additive typically comprises a dispersing agentsuch as stearic acid. The proportion of the said additive in theprecursor is typically less than 2% by weight, and more typicallybetween 0.1 and 1%.

[0015] In this embodiment, the precursor is typically obtained by mixingthe resin, the mineral filler and the additive, and if necessarygrinding the mixture.

[0016] Another object of the invention is a method for coating a givensurface of a substrate with at least one refractory layer containingsilicon wherein:

[0017] the substrate is coated with a coating precursor according to theinvention so as to form a green layer;

[0018] a heat treatment called calcination treatment is carried out toeliminate volatile materials, to calcine the said green layer and toform a cohesive refractory layer.

[0019] The applicant has observed that the method according to theinvention can give a strong thin layer bonding strongly to the substratethat has good resistance to liquid metal and that has good cohesiveness.

[0020] The substrate may be coated (typically including the depositionand spreading of the said precursor on the substrate) by any knownmeans, and preferably by electrostatic powder sprinkling. Thetemperature of the substrate may possibly be increased above the ambienttemperature before coating in order to facilitate the formation of ahomogenous deposit and bonding of the deposition by melting of theresin.

[0021] The method according to the invention may also comprisecomplementary operations such as preparation of the parts of thesubstrate surface to be coated and/or drying of the green coating beforethe heat treatment. The preparation of the substrate surface typicallyincludes cleaning and/or degreasing (for example using acetone).

[0022] The so-called calcination heat treatment comprises at least onestep at a high temperature, typically between 650 and 1300° C., and moretypically between 800 and 1300° C., capable of transforming the greenlayer into a refractory ceramic, that is advantageously in the vitreousstate. The composition of the vitreous phase typically comprises between5 and 25% by weight of silica obtained from the resin (the remainder,typically 75 to 95% by weight, consists essentially of the mineralfiller). The calcination temperature also depends on the substrate; forexample, in the case of a metallic substrate, it is advantageously lessthan the softening temperature of the substrate. Furthermore, it is alsopreferable to use a calcination temperature greater than the workingtemperature of the coated substrate. The heat treatment may include anintermediate step at a temperature of between 200 and 600° C. (typicallybetween 200 and 250° C.). This intermediate step is preferably capableof causing crosslinking of the resin, and possibly decomposition of theresin, before “ceramisation” (or final calcination) of the coating. Inthis case, it is possible, according to an advantageous variant of theinvention, to continue in situ calcination heat treatment, in otherwords when using the substrate at high temperature (preferably higherthan 650° C.).

[0023] The duration of the heat treatment is preferably such that itenables complete ceramisation of the precursor. The temperature increaseis advantageously sufficiently slow to prevent the coating fromcracking.

[0024] During the heat treatment, the organic compounds are eliminated(by evaporation and/or by decomposition) leaving a refractory solid on asurface of the substrate. For example, this solid may be formed frommetal originating from the metal compound and silicon originating fromthe silicone resin. In the case of alumina, silanol groups Si—OH of thepolysiloxane seem to create covalent links with the OH groups ofalumina, the said links seem to transform into Si—O—Al links withrelease of water, during the heat treatment to form an aluminosilicate,which is advantageously in the vitreous state. A similar mechanism mayoccur with metal compounds other than alumina.

[0025] The ambient atmosphere during the calcination treatment isadvantageously non-oxidizing, particularly to prevent oxidation of thesubstrate at the substrate-coating interface that could cause decohesionbetween the substrate and the coating, or even destruction of thesubstrate (for example when the substrate is made of graphite).

[0026] The final coating may comprise two or more successive layers thatmay be applied by coatings and successive heat treatments, i.e. bysuccessive coating/heat treatment sequences. In other words, the layercoating and calcination treatment operations are repeated for eachelementary layer in the final coating. The successive layers may have adifferent composition, such that they have different chemical andmechanical properties. This variant provides a means of adapting eachlayer to a local function, such as bond to the substrate for the firstlayer, mechanical strength for intermediate layers and chemicalresistance for the surface layer.

[0027] Another object of the invention is a substrate in which at leastpart of the surface comprises at least one refractory layer obtainedusing the said precursor or using the said coating method, the saidrefractory layer being advantageously in the vitreous state, with orwithout a gradient of the composition in the direction perpendicular tothe surface of the substrate.

[0028] Another object of the invention is the use of the said precursoror the said coating method for the protection of a substrate,particularly for the protection of a material and/or a piece ofequipment that will be exposed to an oxidising environment, or liquidmetal (particularly aluminium, an aluminium alloy, magnesium or amagnesium alloy, in the liquid state) and/or a solid or molten salt.

[0029] The term “substrate” must be understood in the broad sense: thesubstrate may be made of metal (such as an iron-nickel-chromium basedalloy (typically a steel or inconel)), or a refractory material or acarbonaceous material (such as graphite), or a mixture or a combinationthereof; it may be a particular object (typically a piece of equipment,such as a metallic or refractory component of a casting unit, a nozzle,a liquid metal distributor in a sump, a screen made of steel(particularly stainless steel) or a refractory or ceramic material, ametallic or refractory filter, a liquid metal or gas bubbles injector, arotor, a scraper, a pouring spout, an ultrasound sensor, a measurementsensor (ultrasound, temperature, etc.) designed to be immersed in liquidmetal, parts made of carbonaceous materials, bricks made of graphite,etc.), or a material, particularly a coating material (such as a brickmade of a refractory material or a carbonaceous material (such asgraphite)). The substrate may be porous or non-porous.

[0030] Tests

[0031] Several tests have been carried out on different substrates.These tests were carried out using the following components:

[0032] Mineral Fillers:

[0033] calcined alpha alumina powders (alumina ref. P152SB and AC44 madeby the Aluminium Pechiney company) with a D₅₀ of 1.5 μm and 50 μmrespectively and a BET specific surface area of 3 and 1 m²/g,respectively;

[0034] a TiB₂ powder (reference ESK type S) with a D₅₀ of 45 μm;

[0035] Silicone resin: a MK polymethylsiloxane made by the Wackercompany which is a trifunctional resin with about 1% of OH groups. Thisresin was composed of about 80% of silica equivalent and 20% of methylgroups, which dissociate at a temperature of the order of 450° C.;

[0036] Powder compositions were tested. They had the followingcompositions (% by weight): 85.25% of mineral filler (alumina or TiB₂),14.49% of silicone resin and 0.26% of stearic acid as an additive toreduce the viscosity of the mixture. The proportions were such that therefractory coating obtained comprised about 88% by metal compoundequivalent weight (or mixture of metal compounds) and 12% by silicaequivalent weight.

[0037] The powders were prepared with equipment known in plastictechnologies, including a mixer. A composition based on 100 g of fillerwas added into this mixer, preheated to 100° C. so that work could bedone above the resin melting point and below the resin crosslinkingtemperature. At this temperature, the resin melted and was intimatelymixed with the filler. A hard block was obtained after cooling. Thisblock was ground, firstly with a jaw crusher down to a size grading of 1mm, and then with a ball mill until the size grading was smaller than150 μm.

[0038] The powders obtained were deposited by electrostatic powdersprinkling on different substrates, such as nozzles and grids made of304 L stainless steel.

[0039] The coated substrates were crosslinked at a temperature of 240°C. for one hour.

[0040] The final thickness of the coating was typically of the order of50 μm for a layer. This coating was very homogenous and strong (withgood cohesiveness and non-powdering) and did not block the openings ofgrids when they were used.

[0041] Substrates thus coated were dipped directly in liquid aluminiumat a temperature of about 710° C. Ceramisation was done in situ.

[0042] No degradation to the coating was observed after several hours,or even several days, of immersion.

1. Coating precursor comprising a silicone resin and a mineral fillercapable of chemically reacting with the said resin so as to produce acohesive refractory layer after a calcination operation.
 2. Coatingprecursor according to claim 1, characterised in that the siloxanicpatterns of the silicone resin include trifunctional or quadrifunctionalpatterns.
 3. Coating precursor according to claim 1, characterised inthat the silicone resin is a polysiloxane comprising a proportion of OHgroups.
 4. Coating precursor according to claim 3, characterised in thatthe said polysiloxane is a polymethylsiloxane, a polydimethylsiloxane, apolymethylsilsesquioxane, or a mixture thereof, comprising a proportionof OH groups substituted for methyl groups.
 5. Coating precursoraccording to claim 3, characterised in that the proportion of OH groupsis between about 0.5% and about 2%.
 6. Coating precursor according toclaim 1, characterised in that the mineral filler is chosen from amongmetal oxides, metal and non-metal carbides, metal and non-metal boridesand metal and non-metal nitrides, or a combination or a mixture thereof.7. Coating precursor according to claim 6, characterised in that themineral filler comprises a calcined alpha alumina.
 8. Coating precursoraccording to claim 6, characterised in that the mineral filler is chosenfrom the group comprising ZrO₂, ZrB₂, TiB₂, TiO₂, boron nitride, and amixture or a combination thereof.
 9. Coating precursor according toclaim 1, characterised in that the mineral filler is in the form of apowder with the size of the grains between 1.5 μm and 100 μm. 10.Coating precursor according to claim 1, characterised in that theproportion of silicone resin in the precursor is between 10 and 20% byweight.
 11. Precursor according to claim 1, characterised in that theproportion of mineral filler in the precursor is between 80 and 90% byweight.
 12. Precursor according to claim 1, characterised in that italso includes an additive capable of reducing the viscosity of theprecursor.
 13. Precursor according to claim 12, characterised in thatthe additive comprises a dispersing agent such as a stearic acid. 14.Precursor according to claim 12, characterised in that the proportion ofadditive in the precursor is less than 2% by weight.
 15. Precursoraccording to claim 12, characterised in that the proportion of additivein the precursor is between 0.1 and 1% by weight.
 16. Precursoraccording to claim 1, characterised in that it is in the form of apowder.
 17. Method for coating a given surface of a substrate with atleast one refractory layer containing silicon wherein: the said surfaceis coated with a coating precursor according to claim 1, so as to form agreen layer; a heat treatment called calcination treatment is carriedout to eliminate volatile materials, to calcine the said green layer andto form a cohesive refractory layer.
 18. Method according to claim 17,wherein coating is deposited by electrostatic powder sprinkling. 19.Method according to either claim 17, wherein the temperature of thesubstrate is increased above the ambient temperature before coating. 20.Method according to claim 17, wherein the said calcination treatmentcomprises at least one step at a temperature of between 650 and 1300° C.capable of transforming the green layer into a refractory ceramic. 21.Method according to claim 17, wherein the said heat treatment comprisesan intermediate step at a temperature of between 200 and 600° C. 22.Method according to claim 17, wherein the said calcination treatment iscarried out in a non-oxidising atmosphere.
 23. Method according to claim17, wherein the said refractory layer is formed from several successivelayers.
 24. Method according to claim 17, characterised in that the saidsubstrate is made of metal, a refractory material or a carbonaceousmaterial, or a mixture or a combination thereof.
 25. Method according toclaim 24, characterised in that the said metal is aniron-nickel-chromium based alloy.
 26. Method according to claim 17,wherein the said substrate is chosen from the group comprising metallicor refractory components of a casting unit, nozzles, liquid metaldistributors in a sump, screens made of steel, stainless steel, arefractory material or ceramic, metallic filters, refractory filters,liquid metal injectors, gas bubble injectors, rotors, scrapers, pouringspouts, ultrasound sensors, measurement sensors designed to be immersedin liquid metal, bricks made of refractory material, parts made ofcarbonaceous materials and bricks made of graphite. 27-30 (canceled).31. Substrate characterised in that at least part of the surfacecomprises at least one refractory layer obtained using a precursoraccording to claim
 1. 32. Substrate according to claim 31, characterisedin that it is made of metal, a refractory material or a carbonaceousmaterial, or a mixture or a combination thereof.
 33. Substrate accordingto claim 32, characterised in that the said metal is aniron-nickel-chromium based alloy.
 34. Substrate according to claim 31,characterised in that it is chosen from among the group comprisingmetallic or refractory components of a casting unit, nozzles, liquidmetal distributors in a sump, screens made of steel, stainless steel, arefractory material or ceramic, metallic filters, refractory filters,liquid metal injectors, gas bubble injectors, rotors, scrapers, pouringspouts, ultrasound sensors, measurement sensors designed to be immersedin a liquid metal, bricks made of refractory material, parts made ofcarbonaceous materials and bricks made of graphite.